Recently, a hybrid topology power converter with a three-level inverter and a cascaded H-bridge module was developed so as to provide a novel and high-power topology of multilevel power converter. To avoid the rush current damaging the three-level inverter and the H-bridge module, or damaging the DC bus capacitor when the hybrid topology power converter starts to operate, the DC bus capacitor of the system is pre-charged before the operation of the hybrid multilevel power converter.
In the traditional static Var generator (SVG) with the cascaded H-bridge module, grid connects the SVG via the current-limiting resistor unit and main breaker, and the pre-charge method is performed by turning on the main breaker and allowing the grid voltage to charge the DC bus capacitor of the H-bridge module via the current-limiting resistor unit.
However, in the hybrid topology power converter with the three-level inverter and the cascaded H-bridge module, when the grid voltage to charge the DC bus capacitor of the three-level inverter and the DC bus capacitor of the H-bridge module via the current-limiting resistor unit, the charging speed of the DC bus capacitor of the three-level inverter and the charging speed of the DC bus capacitor of the H-bridge module are different with each other, as the capacitance of the DC bus capacitor of the three-level inverter and the capacitance of the DC bus capacitor of the H-bridge module are different with each other. Consequently, it is difficult to control the voltage of the DC bus capacitor of the three-level inverter and the voltage of the DC bus capacitor of H-bridge module when the pre-charging process is finished, and the close-loop control of the hybrid topology power converter with the three-level inverter and the cascaded H-bridge module isn't easy to be achieved.
Moreover, another traditional pre-charge method is performed by pre-charge devices, which are additionally disposed in the three-level inverter and the H-bridge module to connect with respective DC bus capacitors of the three-level inverter and the H-bridge module. Although the pre-charge resistor unit and the soft-start contactor can be omitted and the voltage of the DC bus capacitor can be controlled by using the above-mentioned method when the pre-charge process is finished, plural pre-charge devices need to be employed additionally for coupling with the DC bus capacitors for pre-charging the DC bus capacitors. Consequently, the control of the hybrid multilevel power converter is difficult and complex.
Therefore, there is a need of providing a pre-charge control method for a hybrid multilevel power converter in order to eliminate the above drawbacks.